In this article

TlsAlloc function

12/5/2018

2 minutes to read

In this article

Allocates a thread local storage (TLS) index. Any thread of the process can subsequently use this index to store and retrieve values that are local to the thread, because each thread receives its own slot for the index.

Syntax

DWORD TlsAlloc(
);

Parameters

This function has no parameters.

Return Value

If the function succeeds, the return value is a TLS index. The slots for the index are initialized to zero.

If the function fails, the return value is TLS_OUT_OF_INDEXES. To get extended error information, call
GetLastError.

Remarks

Windows Phone 8.1: This function is supported for Windows Phone Store apps on Windows Phone 8.1 and later. When a Windows Phone Store app calls this function, it is replaced with an inline call to FlsAlloc. Refer to FlsAlloc for function documentation.

Windows 8.1, Windows Server 2012 R2, and Windows 10, version 1507: This function is supported for Windows Store apps on Windows 8.1, Windows Server 2012 R2, and Windows 10, version 1507. When a Windows Store app calls this function, it is replaced with an inline call to FlsAlloc. Refer to FlsAlloc for function documentation.

Windows 10, version 1511 and Windows 10, version 1607: This function is fully supported for Universal Windows Platform (UWP) apps, and is no longer replaced with an inline call to FlsAlloc.

The threads of the process can use the TLS index in subsequent calls to the
TlsFree,
TlsSetValue, or
TlsGetValue functions. The value of the TLS index should be treated as an opaque value; do not assume that it is an index into a zero-based array.

TLS indexes are typically allocated during process or dynamic-link library (DLL) initialization. When a TLS index is allocated, its storage slots are initialized to NULL. After a TLS index has been allocated, each thread of the process can use it to access its own TLS storage slot. To store a value in its TLS slot, a thread specifies the index in a call to
TlsSetValue. The thread specifies the same index in a subsequent call to
TlsGetValue, to retrieve the stored value.

TLS indexes are not valid across process boundaries. A DLL cannot assume that an index assigned in one process is valid in another process.